Grease seals have been in use for many years to prevent loss of grease from and entry of dirt and liquid into bearings, including ball bearings.
Ordinary elastomers have been used for excluding grit and water, but they have tended to wear out quickly and also to exert excessive friction against their mating member, unless the risk is taken of not providing sufficient interference.
Polytetrafluoroethylene can give longer life and also reduce friction for a given interference but polytetrafluoroethylene is not easy to mold and in the past has been costly. Hence, the normal polytetrafluoroethylene lip is a simple flat wafer type of washer or annulus, and such a lip has been clamped into place.
Even with polytetrafluoroethylene, it is desirable to assure that the more rapid the rotation the better the seal between the lip and the member it engages.